Reactive copying sheet and method of using



United States Patent 3,446,648 REACTIVE COPYING SHEET AND METHOD OFUSING Wesley R. Workman, St. Paul, Minn., assignor to Minnesota Miningand Manufacturing Company, St. Paul, Minn., a corporation of Delaware NoDrawing. Filed Sept. 27, 1965, Ser. No. 490,662 Int. Cl. G03c 1/06 U.S.Cl. 11736.8 5 Claims ABSTRACT OF THE DISCLOSURE Copy-sheets containingreducible silver soap and requiring an image toning agent for makingintense blueblack images contain an acyl derivative of a toning agentand a Weak organic acid, which slowly hydrolyzes to provide the requiredtoning action.

This invention relates to improvements in copypapers useful in thecopying of graphic intelligence by heatinitiated reduction of silvercompounds, and has particu lar reference to improvements in the toningof the silver image.

Heat-sensitive copy-sheets capable of undergoing permanent visiblechange on being briefly heated are described in U.S. Patent No.2,910,377. The color change typically requires the heat-initiatedreduction of a silver compound with an organic reducing agent. Silverbehenate and dihydroxybenzoic acid are representative of reactantsdisclosed in the patent. A similar reaction occurs between a silvercompound and a photosensitive organic reducing agent in the copy-sheetproducts described in U.S. Patents Nos. 3,094,417 and 3,094,619. In allinstances the image formed is intensified and improved by theincorporation, in intimate admixture or association with the silvercompound, of an image toning agent. Although other toning agents for theheat-developed silver image are known, e.g. as described in U.S. PatentNo. 3,074,809, best results in terms of intense blue-black images havebeen obtained with phthalazinone as the toner, as described in U.S.Patent No. 3,080,254.

Under suitably controlled conditions of storage before use, copy-sheetproducts as above described and containing small amounts of phthalazioneas the toner for the silver image are found to retain for long periodstheir ability to form dense black or blue-black images. Under someconditions, and particularly under storage at high humidity and atmoderately elevated temperatures, e.g. as in many tropical orsemi-tropical locations, the imageforming properties of the copy-sheetsare found to deteriorate slowly. Copies made with such sheets, whilestill legible, do not provide the expected dense black images. Thedifiiculty is not effectively overcome by increasing the initial tonercontent, since any excesses in this direction are found to result inunwanted darkening of the sheet at the unreacted background areas.

It has now been found possible to effectually suppress the decay inobtainable image intensity normally occasioned by wet heat aging of thesilver-containing copysheet, by introducing with the phthalazinone toneras a toner progenitor an acyl derivative of phthalazinone which onhydrolysis will yield phthalazinone and an innocuous acid.

An example of an effective toner progenitor is 2-acetylphthalazinone.This compound is readily prepared by refluxing 31.2 g. (0.214 mol.) ofphthalazinone in 175 ml. of acetic anhydride for 2 hours. The mixture iscooled overnight and the solid product is collected on a filter andrecrystallized from benzene. The white solid melts at Patented May 27,1969 Parent acid: Deriv. M.P., C. Lauric 65 Pivalic 99-101 Benzoic 118p-Methozybenzoic 141-143 Ethoxyformic 77-78 The derivatives of oxalicacid (M. 274-275) and of m-nitrobenzoic acid (M. 229-232) are lesseffective. The image formed in the sheet after wet heat aging showslittle if any improvement in appearance over that obtained in theabsence of the derivative. Oxalic acid is known to have a bleaching ordecolorizing effect on many materials. The dissociation constants inwater of oxalic and m-nitrobenzoic acid are orders of magnitude greaterthan those of the others named. The melting point of the oxalyl andm-nitrobenzoyl derivatives is significantly above the temperature of C.frequently considered as an approximate upper temperature limit incommercial thermographic copying processes.

On the contrary, compounds such as 2-phthalyl phthalazinone oiferadditional improvement in toning of the silver image due to theindependent toning action of the acidic component which is obtained,together with phthalazinone, on hydrolysis of the derivative.

The acyl derivatives have the structure 0 N-il R 1 is the acyl radicaland R may be alkyl, aryl, substituted aryl, aralkyl, or alkoxy. ,f.

The several acyl derivatives have very slight if any toning influence onthe silver image as first introduced. They develop toner propertiesduring exposure to wet heat, and at different rates depending on thespecific acyl substituent. Thus, for example, the derivative based onpivalic acid generates a toning action more rapidly than any of theothers named; the acetyl, lauryl and benzoyl derivatives areintermediate in rate of toner generation; 2(3'4'-dimethoxybenzoyl)phthalazinone generates the toner at a relatively slow rate. Since therate of deterioration of the toning action provided by phthalazinoneincorporated in the original coating may also vary extensively dependingon such factors as the specific backing, silver salt, binder, or othercomponents of the sheet, the relative proportions and the extent ofcompatibility of the several components, the storage conditionsemployed, and perhaps other factors, it will be seen that some selectionof particular acyl derivatives may be necessary to achieve the highestorder of uniformity in toner performance or the most extended usefullife of the copy-sheet.

3 Example 1 A master mixture is first prepared as described in US.Patent No. 3,107,174 and containing, in parts by weight, 2.8 parts of amixture of equal mol percent of silver behenate and behenic acid, 2.25parts of terpene resin, 11.25 parts of zinc oxide, 1.69 parts ofpolyvinylacetate as a film-forming binder, and optionally 0.05 part oftetrachlorophthalic anhydride, smoothly dispersed in 77.5 parts ofacetone.

To IO-gram portions of the mixture are then added 0.1 gram of an acylderivative of phthalazinone as a toner progenitor, and the portions aresmoothly knife coated through a 2-rnil coating orifice onto white paperand dried to produce a series of copy-sheets. I

An intermediate sheet with latent image areas containing trace amountsof 4-methoxy-l-hydroxynaphthalene,a reducing agent for silver ion, isplaced against the coated surface and exposed to heat. The resultingoxidationreduction reaction produces a visible image on the copysheetcorresponding to the latent image of the intermediate sheet. Theprocedure is described in more detail in US. Patent No. 3,094,417.Another portion of the coated copysheet is placed in a test chamber andheld at 90% relative humidity and 53 C. for 75 minutes, and is thensimilarly treated to form an image. In all instances the image formed onthe sheet which has been exposed to wet heat is darker and more dense inappearance. The specific derivatives tested are 2-acetyl-, 2-benzoyl-,2- (p-methoxybenzoyl)-, and 2-(3',4-dimethoxybenzoyl)- phthalazinone.

Example 2 To four 10-gram portions of the master mixture of Example 1 isadded .05 gram of 2,6-di-tertiary-butyl-4- methoxyphenol and amounts ofphthalazinone (I) and of the 3,4-dimethoxybenzoyl derivative ofphthalazinone (II) as specified in the following tabulation. Theportions are coated on white paper using a knife coater at an orifice of3 mils, and dried. Portions of the resulting copysheets are subjected towet heat aging at 53 C. and 90% RH. for various times. All portions arethen subjected to heating in contact with an intermediate sheet bearingtrace amounts of 4-methoxy-1-hydroxynaphthalene at latent image areas,to form visible images on the copy-sheets. The image optical density ofeach of the four samples is then determined by reflection. Acommercially acceptable visibly dense image has an optical density valueof at least about .50.

Toner material, Density value after aging for In s (hours)- II 1 4 l6which is itself heat-sensitive and requires no added trace amounts of4-methoxy 1 hydroxynaphthalene or other reducing agent in order toundergo visible darkening when heated briefly at elevated temperaturesof the order of to C. as in thermographic copying processes. Thepresence of the significant small amount of 2-acyl pthala- Zinonetogether with the phthalazinone insures the formation of a dense darkimage both with the sheet as initially prepared and after it has beensubjected to prolonged exposure to high humidity and elevatedtemperature storage conditions. 1

What is claimed is as follows:

1. Sheet material containing, in intimate association, a silver salt ofan organic acid and a significant small amount of 2-acyl phthalazinonehaving the structure wherein o H OR is an acyl radical which isgradually removable by hydrolysis under exposure to wet heat atmosphericconditions and R is alkyl, aryl, substituted aryl, aralkyl, or alkoxy.

2. Sheet material of claim 1 wherein is included a significant smallquantity of phthalazinone.

3. Sheet material of claim 1 which is capable of undergoing permanentvisible change on being briefly heated and including an organic reducingagent for silver ion.

4. Sheet material of claim 3 wherein is included a significant smallquantity of phthalazinone.

5. The method of achieving the toning of silver images formed in acopy-sheet containing a reducible silver salt of an organic acid, comrising including with said silver salt an acyl deriverative of aphthalazinone toner for said silver image, the acyl radical beinggradually removable by hydrolysis under exposure to wet heat atmosphericconditions, and permitting at least partial hydrolysis of saidderivative prior to formation of said image whereby to liberate asignificant amount of said toner.

References Cited UNITED STATES PATENTS 2,910,377 10/1959 Owen l17-36.83,074,809 1/1963 Owen 11736.8 3,080,254 3/1963 Grant 11736.8 3,094,4176/1963 Workman 11736.9 3,094,619 6/1963 Grant 117-36.8 3,107,174 10/1963Wartman 117-36.8 3,218,166 11/1965 Reitter 11736.8

MURRAY KATZ, Primary Examiner.

US. Cl. X.R.

